The present invention relates to an image processing apparatus, a medium, and a method of processing image data. More specifically, the present invention relates to an image processing apparatus configured to execute a process of creating a PDF (Portable Document Format) file from image data using MRC (Mixed Raster Content), a non-transitory computer readable medium storing a program causing a computer to execute the process, and a method of processing image data. A non-transitory computer readable medium includes a hard drive, a compact disc, a flash memory, a volatile memory, and the like, but does not include a transitory signal per se.
A conventional image processing apparatus such as an MFP (Multi Function Product) has been universally available. Such a conventional image processing apparatus may be provided with a function of converting image data (scan data) obtained through reading a text and the like to a PDF (Portable Document Format) file, so that the conventional image processing apparatus outputs data of the PDF file.
In general, information quantity of the image data tends to become larger without compression. Accordingly, when the PDF file is created, the image data are compressed to reduce the information quantity of the image data, thereby alleviating load on the communication network or requirement to a recording medium. Recently, PDF has been widely used and standardized (ISO 32000-1), so that the format has been widely available. As a result, it has been generally possible to express an image with a multiple layers structure.
Patent Reference has disclosed a conventional image expression method using the multiple layers structure. In the conventional image expression method disclosed in Patent Reference, it is possible to efficiently compress a document image generally called MRC (Mixed Raster Content), in which a natural image and a text image coexist, while maintaining image quality.    Patent Reference: Japanese Patent Publication No. 11-177977
Currently, the JPEG method has been widely used as a compression method of an image. The JPEG method is a non-reversible compression method utilizing continuity of an image, and it is possible to reduce the information quantity without excessively damaging image quality of an appearance of a natural image. For this reason, the JPEG method has been frequently used for compressing an image constituting a layer of MRC. However, when the JPEG method is adopted to compress a text color image with an abrupt color change and the like, it is difficult to obtain good image quality and high compression efficiency due to the nature of the JPEG method.
To this end, it has been required to provide an image processing apparatus, a non-transitory computer readable medium storing a program causing a computer to execute a process, and a method of processing image data, which is capable of obtaining a decoded image with good image quality, even when the image data of the image expressed with the multi layer structure (for example, the image expressed with MRC) are processed (compressed, encoded, and the like) with the non-reversible method (for example, the JPEG method).
An object of the present invention is to provide an image processing apparatus, a medium, and a method of processing an image capable of maintaining good image quality.
Further objects and advantages of the invention will be apparent from the following description of the invention.